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Beyond space and time: 10D – String country

26 August 2009

By Anil Ananthaswamy

Ten dimensions, and we finally reach the fabled land of string theory. For all the vitriol that has been thrown at it, string theory is for the moment the only real game in town when it comes to attempts to bundle up quantum mechanics and general relativity into a “theory of everything”. It holds that all particles that make up matter or transmit forces arise from the vibration of tiny strings. Those strings are one-dimensional. The space they wiggle about in is not. In fact, it has 10 dimensions&colon; nine of space, and one of time.

Why? In a nutshell, because the theory doesn’t work with any fewer, as physicists Michael Green and John Schwarz showed in 1984&colon; mathematical anomalies crop up that translate into violent fluctuations in the fabric of space-time at scales smaller than the Planck length of 10-35 metres.

That doesn’t necessarily mean that 10 is the magic number. Indeed, one now unfashionable early variant of string theory had 26 dimensions. There are five broadly defined brands of 10D string theory that compete to explain the universe, with no indication as to which, if any, is the right one. But these disparate theories can be unified into one overarching theory, known as M-theory. M-theory has 11 dimensions.

It is assumed that the extra dimensions of M-theory must in some way be squashed down to a size that we can’t see. The bad news is that there is an almost unlimited number of ways in which this can be done. How to single out the one way that produces our universe remains a …